Thanks for answering Gonzo. The deck is very straight forward. The deck measures 15' x 7' and the deck will have to span no more than 2' in the largest area. The deck is of course supported by bulkheads & stringers and has five cutouts for hatches. I will be able to get to the underside to bond the two together with fiberglass & fillets. It needs to support no more than a few fishermen. The deck that I'm taking out is 3/8" ply with about 1/8" of glass on both sides for a total thickness of 5/8". So what I'm looking for is a deck of comparable strength. i don't think it is anything fancy & I have a good grasp on how to how to build it right. What I don't have is much experience with fiberglass other than doing a lot of reading so I have no bench mark to judge with. I think a little backyard engineering will be just fine for this project. For example will 3/4" H80 covered with 1708 on both sides give me what I'm looking for? By the way the deck will be coated with a textured non skid surface-er when it is finished.
Thank for the help

Please re-read my post. You are not providing enough information. Is this a rectangular deck? What is the camber if any? What is the loads transmitted from the hull into the deck? What vertical loads will the bulkheads put on the deck, if any? What is the distance between bulkheads and carlins? You can look at a similar boat and copy the structure if there is one. Engineering is based on known models.

Thanks again...Ill try to answer as best as i can. The boat is built like a tank 1983 Farallon. The deck is a 15' x 7' rectangle, there 1 1/2" camber? or 1 1/2" fall from the center of the deck to the inside side of the boat or 1 1/2" in 42" and fall from front to back. The inside structure is so beefy that there cant be much if any vertical loads transferred to the deck. The hull is 1/2" thick on the bottom - beefy. The biggest span over support structure would be no more than 2'. Imagine a 2' grid of bulkheads & carlins?

The deck should be considered the upper flange of a beam, as a structural element in the over all hull design. Not only does it carry localized loads but global considerations must be expected. Simply put, the 1708 over Divinycell will not have the same physical attributes as the very heavily sheathed 9 mm plywood used previously. Will you be using polyester, vinylester or epoxy resin? Is this deck going to be tabbed to the hull shell?

Par
The train of thought was to use epoxy for all bonding, primary & secondary. I was planning to tab the deck to the sides, bulkheads and carlins? as I will have access to these areas thru deck hatches. As far as the I beam effect there are four bulkheads in 15' and then there is the transom to tie into. As far as the hatches go i was going to build the frames, which will drop about 4" below the deck adding a degree of rigidity & strength.
Thanks for the help I'm learning a lot here

Strength is a generic term that can be confusing. A sandwich construction with foam can be more rigid than a thinner one with plywood core. However, the foam core with thinner laminate will be less puncture resistant. Sometimes a combination of plywood over foam gives better overall performance. Is there a reason not to use plywood again? It is much easier to use and to fair. You can fiberglass the under side of the panels before installing them, otherwise it will be really hard to do.

Gonzo
The reason for using foam is two fold. The first reason is the weight savings. I'm installing a larger/heaver engine than the boat original had so I would like to off set the weight difference it a bit. I will be saving almost 280lbs in the weight of the deck by using the foam.. The second reason is that I need to raise the height by about 1", it will be much easier to do with a thicker core material.
Thanks

It should be easy to add shims under the deck at bulkheads and carlins. With foam you need to fiberglass the under side too. You may have to make some kind of jig to hold the foam with an approximate camber, so when you turn it over the foam won't split and make hard spots. On the span you are describing, a 1/2" light weight plywood with fiberglass only on the top may save you the same or more weigth

Also its not possible to mechanicaly fasten to a foam core...of course you could plan correctly and block up all fastening points in the core. .... but experience has taught me that in future you will always need to fasten something unplanned. Okume Plywood is nice to use , forgiving of unplanned installations and 2 sheets ,at 30 or 40 lbs per sheet, is not so heavy.

Gonzo
Yes the idea was to build a jig to fiberglass one side then flip & glass the other side. I would feel a lot more comfortable with glass on both sides. Then the panels would be glued down to the support structure. I tried looking for light weight plywood but the only stuff I found was only a few pounds lighter than standard marine ply (hardly worth the trouble) besides I really would like to glass both sides.

No need to fasten anything to the deck besides 1 chair at the helm, ply will of course be used here. I have never needed to screw anything to the deck, the sides, cabin roof ....etc yes but not the deck so I'm not worried about fasteners. So do I pass the test on planning? ...and the bigger question is using a foam core on the deck appropriate or should I just let it go?

Do what a friend of mine did ! found a deck mould almost what he wanted off the simular sized boat and made a complete new fibreglass deck ,Gel coat , glass and core and glass etc etc and the spliced it onto his wooden hull .Now its almost finished looks like a completely glass boat !! Did a nice detailed join to the wooden hull using epoxy glued and epoxy glassed both sides for the hull to deck join inside and out and found a aluminium deck edge exstrusion off a racing boat that will hold snap on genoa blocks etc and is through bolted!!
Same with Bulkheads all done with epoxy !! Has a reall nice and really practical 36 foot boat !!

Made a deck for a smaller (22ft) power boat with just light flat fibre glass sheet of 450 csm 5 mm corematt and 450csm ,hot glued together laid and stuck together over a light plywood frame system ,once glassed then cored and glassed outside , it looked like a proper deck out of a mould but was just a one off total glass . knocked all the frame out when it was finished glassed the underside and the owner was wrapped to have 100% glass boat and no wood any where .
Even made a 600mm exstension outboard bracket also out of glass !!, hollow so added to bouyancy in the rear end . Was angled and had two bult in steps each side to be able to get out of the water much more easely . Didnt want a full width transom boarding plateform so did a compromise and the best of both !!.